The overall goal of this project is to establish and validate biomarkers associated with the risk and progression of late onset Alzheimer's disease, mild cognitive impairment and cognitive decline. The investigators will use baseline and longitudinal measurements of plasma amyloid beta-40 and amyloid beta-42 to investigate the risk of developing mild cognitive impairment and late onset Alzheimer's disease, as well as the rates of cognitive decline and Alzheimer's disease progression. Participants will be selected on the basis of change in plasma amyloid beta levels over prior assessment intervals. The purpose of the study is to examine whether brain amyloid plaque load, which will be measured with positron emission tomography and x-ray computed tomography brain imaging using Florbetaben from Bayer, varies as a function of change in plasma levels of amyloid beta. The driving hypothesis of the study is that high plasma levels of amyloid beta are an antecedent indicator of increased risk of cognitive decline, mild cognitive impairment, and incident late onset Alzheimer's disease, and that declining plasma levels of amyloid beta are associated with the onset of cognitive decline. Further, high plasma levels of amyloid beta are related to increased levels of amyloid beta in the brain as measured by positron emission tomography positivity, and the specific pattern of positron emission tomography positivity and a decline in plasma amyloid beta over time are associated with the onset of cognitive decline associated with late onset Alzheimer's disease.